An important function of the vertebrate kidney is the excretion of potentially toxic chemicals, such as, waste products of cellular metabolism, xenobiotics and xenobiotic metabolites. This occurs primarily in the proximal tubule, where specific renal transport systems remove organic anions and organic cations from peritubular capillaries and move them across the tubular epithelium into the lumen. We are using comparative renal models (proximal tubules from lower vertebrates and invertebrates and mammalian renals cells in culture) in combination with fluorescence microscopy (conventional and confocal), video imaging, intracellular microinjection and isolated membrane vesicle techniques to define the cellular mechanisms that underlie xenobiotic excretion. We have found that organic anions and cations are distributed over both diffuse and vesicular compartments within kidney, liver and choroid plexus cells. In proximal tubule and choroid plexus, xenobiotic- containing vesicles move through the cytoplasm in the secretory direction on microtubules; such movement may contribute substantially to net secretion. In addition, imaging studies have provided the first demonstration in intact renal tubules of xenobiotic (cyclosporin and daunomycin) secretion mediated by the multidrug resistance (MDR) transporter and of organic anion secretion mediated by an active, Na- independent system. Finally, pharmacological studies show that all four xenobiotic transport systems in proximal tubule are under control of protein kinase C (PKC). Future work will focus on: 1) characterizing vesicular transport mechanisms driving xenobiotic secretion in proximal tubule and choroid plexus, especially interactions with cytoskeletal and plasma membrane proteins, 2) characterizing the Na-independent organic anion transport system and determining its physiological role in xenobiotic secretion, and 3) identifying the physiological signals that regulate xenobiotic excretion through protein kinase C.